The invention relates generally to an electrical machine, and more particularly to a rotor of an electrical machine, such as a motor and/or generator. Specifically, embodiments of the present technique provide improved loss characteristics in an electrical machine.
Typically, an electrical machine includes a rotor and stator that are utilized to convert electrical power to mechanical power or vice versa. The stator generally includes a large number of laminations of a magnetic material that are stacked together. Within the center of the laminations, a central opening is formed throughout the length of the stator. The rotor may also include magnetic materials that are mounted on a shaft, which is disposed within the central opening of the stator.
The stator may include coils or windings that form magnetic poles in the stator, while the rotor may also include windings or be a solid portion of magnetic material. For instance, the stator may include windings that are located adjacent to the central opening of the stator, while the rotor may include windings that are located on the outside of the rotor adjacent to the stator.
Generally, a time-varying magnetic field is capable of producing eddy currents in electrically conductive materials, such as those from which the stator and rotor are formed. The magnetic metals usually exhibit a relatively high electrical resistivity. Because eddy current formation primarily depends on electrical and magnetic properties of the rotor surface, the magnetic fields produced around the rotor induce eddy currents in the rotor. These eddy currents dissipate energy via resistive heating in the rotor that increases the temperature of the rotor. As a result, eddy currents reduce the efficiency of the electrical apparatus and present thermal problems for the electrical apparatus. Further, the transient behaviors of the electrical machine are, in part, dictated by the rate at which induced eddy currents decay in the rotor surface. Thus, these eddy currents may impact the operation of the electrical machine.
To reduce the heat from the eddy currents, the electrical and magnetic properties of the rotor surface may be modified by attachments to address these problems. However, such attachments may not be advantageous in electrical machines operating at high speeds, such as speeds exceeding 10,000 revolutions per minute because the centrifugal forces and centrifugal stress imposed on such attachments by rotation of the rotor may cause the attachments to disintegrate from the rotor.
Accordingly, there exists a need for controlling rotor magnetic and electrical properties, especially near the surface of the rotor, while providing a desirable mechanical strength for use in high-speed electrical machines.